Electron gun for use in a cathode ray tube exhibiting enhanced electron emission



Nov. 28, 1967 J. w. COLEMAN ELECTRON GUN FOR USE IN A CATHODE RAY TUBE EXHIBITING ENHANCED ELECTRON EMISSION Filed Oct. 21, 1965 I INVENTOR.

JZV/r/ld (aim/4m 'EMJ United States Patent ELECTRON GUN FOR USE IN A CATHODE RAY TUBE EXHIBITING ENHANCED ELECTRON EMISSIGN John W. Coleman, Willingboro, N..l., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 21, 1965, Ser. No. 499,573 8 Claims. (Cl. 313-85) ABSTRACT OF THE DISCLOSURE In a cathode ray tube, it is advantageous, for focusing purposes, to use a point source of electrons as a cathode. However, the thermionic emission from a point source of electrons may not be sufficient for cathode ray tube work. Means are provided to enhance the electron emission of a thermionic cathode by field emission and still produce an electron beam that can be focused.

This invention relates to apparatus for increasing the electron emission of the cathode of a cathode ray tube, such as is used in an electron microscope, while maintaining the focusability of the beam produced from the cathode.

Electrons may be produced by a cathode comprising part of a cathode ray tube either thermionically, that is by boiling them out of the cathode material, or by field effeet, that is by drawing them out of the cathode material by applying a high potential gradient between the cathode and an anode in the vicinity of the cathode. The practical limit of thermionic emission is about 1.8 amperes per sq. cm., which may be too low for cathode ray tube work, since the limiting current density in purely thermionic emission makes the maximal source brightness too low if the electron source is a point. Yet for ease in focusing, the cathode must be substantially a point source. The limit of field emission is very high, about a hundred thousand amperes per sq. cm., which is more than sufficient, however a cathode operated at such emission would burn up after only a few seconds of operation at usual laboratory vaeua. A practical way to produce sufiicient emission for a cathode ray tube from a cathode having a practical length of life, that is, about 30 amperes per sq. cm. for at least 24 hours, is to combine thermionic and field emission. This involves providing a voltage gradient of between 10,- 000 and 15,000 volts per millimeter between the anode and the cathode, and this involves bringing the anode to within about of an inch from the cathode where the range of anode to cathode voltage is 50 to 100 kilovolts. In electron microscopes, the hole through the anode is about /2 inch in diameter, and it the anode is brought to within of an inch from the grid surrounding the cathode, the distance between the cathode and the edges of the hole through the anode is much greater than of an inch, whereby, with a one-half inch hole through the anode, it is impossible to bring the anode close enough to the cathode to provide the required voltage gradient. If the hole in the anode is made small enough so that the desired voltage gradient is produced between the anode and oathode, the electron lens effect of the anode on the electron stream is such that the beam produced by such an electron gun is not focusable on the specimen.

It is an object of this invention to provide an improved electron gun system for a cathode ray tube.

It is an object of this invention to provide an improved electron gun system including a thermionic point cathode which produces a foeusable beam of increased intensity in which the thermionic electron emission of the cathode is increased by an applied field.

It is another object of this invention to provide an improved anode for such an electron gun system.

In accordance with this invention, an anode is provided for an electron gun including a cathode, the anode having a generally conically shaped portion with a hole therethrough which may be conical. It the hole is conical, the smallest diameter part of this hole through the anode should occur at the tip of the anode. The cathode, which may be a point type cathode, is substantially surrounded by a grid having a hole therethrough through which electrons from the cathode may pass. The smallest diameter portion of the hole through the anode is between 3 and 4 times the diameter of the hole through the grid. The anode is so arranged that the tip of the cone-shaped portion thereof is close enough to the cathode so that the voltage gradient between the tip of the cone-shaped portion and the cathode is of the desired magnitude. Furthermore, the exterior of the cone-shaped portion is shaped to provide a field that collimates the beam by lens action so that the beam goes through the hole in the anode and so that it is focusable on the specimen beyond the anode. The exterior of the cone-shaped portion may also be shaped so that any ions that are produced during the operation of the electron gun are guided away from the cathode whereby they cannot bombard and thus harm it.

For further explanation, reference is had to the following description taken with the sole figure of the drawing, which illustrates a sectional view of a cathode ray gun comprising an embodiment of this invention.

In the sole figure, a cathode 10 is shown which comprises a conical tip 12, which is usually less than 15 thousandths of an inch long, of electron emissive material, such as tungsten, welded to the bent portion of a V-shaped thermionic heater 14 comprising a resistive wire which may also be tungsten. The legs of the heater 14 are welded to the respective bases 20 of two L-shaped leads 16. The L-shaped leads 16 are positioned so that the legs 18 thereof are parallel and so that their bases 20 are in line and extend towards each other. A control electrode or grid 22 substantially surrounds the tip 12, and the heater 14 and parts of the lead 16. The grid 22 is made of conductive material and is of the shape of a hollow frustrurn of a cone having a parallel base 24 and 26 having thin walls. A pair of holes 28 are provided in the larger base 24 of the grid 22 through which the legs 18 of the leads 16 extend. The leads 16 are supported on the larger base 24 by individual buttons 30 which insulate the legs 18 from the grid 22. A hole 32 is provided in the center of the smaller base 26. The cathode 10 is so arranged that the tip 12 is concentric with the hole 32 and extends, when hot, into the plane of the outside surface of the base 26, or not more than about .002 of an inch behind it. The axis of the tip extends though the center of the hole 32. Leads 33 are provided to apply a high negative biasing potential to the cathode 10 with respect to ground and to provide a heating current for the heater 14. A lead 34 is provided to bias the grid 22 more negative with respect to ground than the cathode.

An anode 36 is provided comprising a plate 35 having a projection or boss 38 extending from one side thereof. The boss 38 which has a hole 40 therethrough, is symmetrical about its axis. The boss is so formed that all sections thereof perpendicular to its axis are circular annuli, the hole through successive annuli increasing in diameter in a linear manner as the section is taken successively closer to the plate 35. The outer diameter of the successive annuli, however increases in diameter at a progressively greater rate as the sections are taken progressively closer to the plate, up to point 41, whereby a section of the boss through the axis thereof exhibits concave outer boundaries to the point 41, the boundaries of the hole 40 being straight. The boss 38 is necked between the top of the plate and the point 41. With the exception that the anode 36 must not present sharp edges, the presence, the position and the shape of the neck portion is unimportant. The hole 40 is made conical so that the beam from the cathode 10, which expands as it goes downward as viewed in the figure, will clear the boss 38. The smallest diameter of the hole 40 is about 3 to 4 times the diameter of the hole 32. The anode 36 is grounded at 44 as indicated.

An evacuable chamber (not shown) surrounds the grid 22 and the anode 36. The anode 36 and the grid 22 are so positioned in and supported by the chamber that the pin 12 and the boss 38 are coaxial with the tip of the pin 12 close to, that is spaced about of an inch from the small end of the boss 38. The electron beam, after it goes through the anode 36, goes through a hole in an electromagnetic lens (not shown) and through a specimen (not shown) to provide an amplified image of the specimen.

In operation, the cathode 10 is heated by currents passing through the leads 33 and the cathode 10 is placed at a high negative potential with respect to ground by a voltage applied thereto by one of the leads 33. The grid 22 is placed at a higher negative potential with respect to ground by a voltage applied thereto by a lead 34. Since the anode is grounded, a potential gradient exists between the anode 36 and the heated cathode 10 to enhance electron emission therefrom. The small size of the hole 32 through the grid 22 and the negative potential on the grid 22 with respect to the cathode 10 forms these electrons into a beam. Due to the close proximity of the end of the boss 38 to the pin 12, and due to the small size of the smaller end of the hole 40 through the boss 38, a high potential gradient is provided between the anode 36 and the pin 12, whereby the emission from the pin 12 is greatly enhanced. Due to the shape of the boundaries 42 of the upper half of the exterior of the boss 38 (as viewed in the figure) the maximum field gradient is along the axis and along the beam whereby the electrons of the beam are accelerated. Also due to the shape of the outside boundaries 42 of the upper half of the boss 38, the equipotential surfaces between the grid 22 and the boss 38 are so shaped as to collimate the beam and cause it to pass through the anode whereby it can be readily focused by a suitable magnetic lens (not shown) beyond the anode 36.

While the exterior shape of the lower half of the boss 38 is not so important as that of the upper half as far as collimation and acceleration are concerned, the lower half of the exterior of the boss 38 is so formed as to provide a field that guides any ions that may be formed during the operation of the described device away from the pin 12, whereby the ions do not hit the pin 12 and therefore do not injure it. Furthermore, the relatively low resistance path produced by the sheath of ionized particles around the beam between the anode 36 and the grid 22 provides a leakage path which tends to prevent build up of potential between the anode 36 and the grid 22 to a value that could cause disruptive discharge therebetween. This low resistive path also guides the disruptive discharge, when it occurs, from the anode 36 to the grid 22 and away from the cathode 10, whereby the disruptive discharge does not harm the electron gun.

The above description and illustration of the apparatus embodying the present invention are to be considered as only illustrative since many variations thereof within the spirit of the invention are possible.

What is claimed is:

1. An arrangement for increasing the electron emission from a thermionic electron source while maintaining the beam shape of the electrons produced by said source comprising:

an electron emissive cathode means of pin shape and having an axis for providing an electron beam,

an anode having a generally conically shaped boss portion having an axis, there being a hole through said boss portion to provide a passage for electrons from said cathode, the axis of said cathode means and of said boss portions being coincident,

a portion of the exterior of said boss portion being shaped to cause the maximum voltage gradient between said cathode and said anode, when energized, to be along said axis, and also to provide a lens action to collimate said beam.

2. An arrangement for increasing the electron emission from a thermionic electron gun while maintaining the beam shape of the electrons produced by said gun comprising.

an electron emissive cathode means for providing an electron beam,

an anode having a generally conically shaped boss portion having an axis, the small end of said anode being towards said cathode, there being a hole through said boss portion to provide a passage for electrons from said cathode, said hole being conical and being coaxial with the axis of said anode,

a portion of the exterior of said boss portion adjacent said cathode means being shaped to cause the maximum voltage gradient between said cathode and said anode, when energized, to be along said axis, and also to provide a lens action to collimate said beam, and

another portion of the exterior of said boss portion remote from said cathode means being shaped to cause ions produced during ope-ration of said electron gun to be directed away from said cathode, the shape of sections of said boss portion through said axis exhibiting concave outer boundaries.

3. An arrangement for increasing the electron emission from a thermionic electron gun While maintaining the beam shape of the electrons produced by said gun comprising:

an electron emissive pin having an axis,

. means for heating said pin to an electron emitting tern perature,

a hollow beam forming grid electrode substantially surrounding said pin, there being a hole in said grid in line with the axis of said pin through which electrons from said pin pass to form an electron beam,

an anode having a generally conically shaped boss portion having an axis, there being a hole through said boss portion to provide a passage for electrons from said pin, said pin and said boss portion being coaxial,

a portion of the exterior of said boss portion being shaped to cause the maximum voltage gradient between said pin and said anode, when energized, to be along said axis, and also to provide a lens action to collimate said beam, and

another portion of the exterior of said boss portion being shaped to cause ions produced during operation of said electron gun to be directed away from said pin.

4. An arrangement for increasing the electron emission from a thermionic electron source while maintaining the beam shape of the electrons produced by said source comprising:

an electron emissive cathode means for producing an electron beam,

an anode having a generally conical boss portion having an axis and having a small and extending towards said cathode means, there being a hole through said boss portion to provide a passage for electrons from said cathode, each section of said boss portion perpendicular to said axis being an annulus having a circular outer and inner boundary, the diameter of said outer boundary increasing in a direction from the small end of said boss towards the larger portion of said boss, said diameter increasing in such a man ner that a section of said boss through said axis. exhibits concave outer boundaries.

5. An arrangement for increasing the electron emissTon from a thermionic electron gun while maintaining the beam shape of the electrons produced by said gun com prising:

an electron emissive pin having an axis, a section of said boss through said axis exhibits means for heating said pin to an electron emitting concave outer boundaries and straight inner boundternperature, aries, the axis of said pin being in line with the axis a hollow beam forming grid electrodes substantially of said boss portion.

surrounding said pin, there being a hole in said grid 5 7. An anode for a thermionic electron gun said anode in line with the axis of said pin through which eleccomprising: trons from said pin pass to form an electron beam, a generally conical. boss portion consisting entirely of an anode having a generally conical boss portion havconductive material having an axis and having a ing an axis and having a small end extending tosmall end with a hole through said boss portion,

wards said pin, there being a hole through said boss 10 each section of said boss portion perpendicular to portion to provide passage for electrons from said said axis being an annulus having a circular outer pin, each section of said boss portion perpendicular and inner boundary, the diameter of said outer to said axis being an annulus having a circular outer boundary increasing in a direction from the small and inner boundary, the diameter of said outer end of said boss towards the larger portion of said boundary increasing in a direction from the small 1 boss, said diameter increasing in such a manner that end or" said boss towards the larger portion of said boss, said diameter increasing in such a manner that a section of said boss through said axis exhibits concave outer boundaries.

a section of said boss through said axis exhibits concave outer boundaries.

8. An anode for a thermionic electron gun said anode comprising:

6. An arrangement for increasing the electron emisa generally conical boss portion having an axis and sion from a thermionic electron gun while maintaining having a small end with a conical hoie through said the beam shape of the electrons produced by said gun boss portion, each section of said boss portion percornprising: pendicular to said axis being an annulus having a an electron emissive pin having an axis, circular outer and inner boundary with the diameter means for heating said pin to an electron emitting of said inner boundary increasing in adirection from temperature, the small end of said boss toward the larger portion a hollow beam forming grid elecrodes substantially surof said boss, the diameter of said outer boundary rounding said pin, there being a hole in said grid also increasing in a direction from the small end in line with the axis of said pin through which eiecof said boss towards the larger portion of said boss,

trons from said pin pass to form an electron beam, said diameters increasing in such a manner that a an anode having a generally conical boss portion havsection of said boss through said axis exhibits coning an axis and having a small end extending tocave outer boundaries and straight inner boundaries.

wards said pin, there being a conical hole through said boss portion to provide passage for electrons References Cited from said pin, each section of said boss portion per- UNITED STATES PATENTS pendicular to said axis being annulus having a circular outer and inner boundary, the diameter of sdchlgsmger said inner boundary increasing in a direction from 5 11/1944 Her "5 the small end of said boss toward the larger por- 7/1 62 amo 3 tion of said boss, the diameter of said outer bound- 9 9 Houston 313*336 ary also increasing in a direction from the small end of said boss towards the larger portion of said boss, said diameters increasing in such a manner that JAMES W. LAWRENCE, Primary Examiner. V. LAFRANCHI, Assistant Examiner. 

1. AN ARRANGEMENT FOR INCREASING THE ELECTRON EMISSION FROM A THERMIONIC ELECTRON SOURCE WHILE MAINTAINING THE BEAM SHAPE OF THE ELECTRONS PRODUCED BY SAID SOURCE COMPRISING: AN ELECTRON EMISSIVE CATHODE MEANS OF PIN SHAPE AND HAVING AN AXIS FOR PROVIDING AN ELECTRON BEAM, AN ANODE HAVING A GENERALLY CONICALLY SHAPED BOSS PORTION HAVING AN AXIS, THERE BEING A HOLE THROUGH SAID BOSS PORTION TO PROVIDE A PASSAGE FOR ELECTRONS FROM SAID CATHODE, THE AXIS OF SAID CATHODE MEANS AND OF SAID BOSS PORTIONS BEING COINCIDENT, A PORTION OF THE EXTERIOR OF SAID BOSS PORTION BEING SHAPED TO CAUSE THE MAXIMUM VOLTAGE GRADIENT BETWEEN SAID CATHODE AND SAID ANODE, WHEN ENERGIZED, TO BE ALONG SAID AXIS, AND ALSO TO PROVIDE A LENS ACTION TO COLLIMATE SAID BEAM. 